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TekSpek 's
RAID
Date issued:
RAID 4: Block level Striping with Dedicated Parity Drive
A commonly used implementation of RAID before the advent
of RAID 5, RAID 4 provides block-level striping (like RAID 0) with
a parity disk. If a disk fails, the parity data is used to create
a replacement disk. Parity is used to provide redundancy without
the overhead costs involved in a mirrored array, whereas RAID 1
utilizes 50% of the total capacity the principle of parity is to
take “X” amount of data and use that to compute an extra
piece of data, then these “X+1” pieces of data are taken
and stored on “X+1” drives. If any one of these pieces
of data is lost then it can be restored from the data that remains.
With parity, if you use a 4 disk array then you have the effective
space of 3 disks, whereas with mirroring you would only get the
equivalent of the space from 2 disks.
Parity protection is used with a RAID 0 (striped) array and “X” is normally the blocks or bytes distributed across the array. The parity data can be either on a dedicated parity drive as with RAID 4 or spread amongst the drives as in RAID 5.
Parity has some obvious advantages over mirroring in overhead costs, mirroring has a 50% overhead for its redundancy whereas parity has an overhead of 100/D where D = the number of drives in the array. As parity is used with a striped array the performance benefits of striping are also apparent.
The complexity of the millions of calculations that have to be performed every second leads to a major disadvantage with parity. Additional processing power is required leading to the necessity of a hardware controller for high performance. Software RAID with striping and parity utilizes a large amount of CPU power and slows the system as a whole. Similarly, should a drive fail the missing data has to be reconstituted, again requiring millions of calculations which takes time. A mirrored array is quick and simple to recover from particularly if hotpluggable drives are used. The use of a single parity drive can also create a bottleneck in read/write speeds slowing overall performance.
